Assigning Slots in a Mesh Network

ABSTRACT

Assigning slots to nodes in a mesh network. Slot numbers are assigned to nodes in a wireless mesh network using a depth-first search combined with information on 2-hop neighborhoods for each node. Assigning slots using 2-hop neighborhood information allows slots to be safely reused. The slot assignment process may take process in parallel using different wireless channels for different subtrees rooted to a controller. Slot assignment may be repeated when the mesh topology changes. Reporting using the slot numbers allows for information from child nodes to be aggregated or filtered at parent nodes.

BACKGROUND OF THE INVENTION

The present invention relates to wireless systems, and in particular, to the problem of assigning reporting slots to nodes on a wireless mesh network.

Wireless networks, such as those operating according to IEEE 802.11 standards typically provide wireless packet-based data services to clients in a network. These clients may include computers such as laptops, hand-held devices such as smart phones, scanners, and the like, as well as wireless infrastructure devices such as sensors and cameras. In one common implementation, a plurality of wireless access nodes, each typically a small purpose-built computer system with one or more wireless interfaces, also includes a wired interface such as IEEE 803.2 Ethernet, to connect the access node back to a controller, which provides access to network services, as well as optionally providing functionality such as authentication, security, advanced mobility, and the like. In such a network, clients establish a wireless connection with an access node. Traffic to and from the client is passed to and from the access node wirelessly, and to and from the access node and the controller through the wired connection.

There are circumstances in which it may be difficult or expensive to run a data connection between each wireless access node and a controller. Mesh networks are useful in such circumstances. In a mesh network, rather than having each node wired to a controller, wireless access nodes establish wireless connections with each other, with only a small number of access nodes having wired access to a controller. In such a mesh network, clients also establish a wireless connection with an access node. But rather than traffic going wirelessly from client to access node, and from there by wired connection to the controller, in a mesh network, traffic first passes wirelessly from client to a first access node, and then wirelessly from that first access node through one or more other access nodes in the mesh network, until the traffic arrives at a root node or mesh portal, a wireless node which also has a wired connection to the controller, and from there to the controller. The mesh connection between access nodes may be through the same radio system and/or same radio channels as used by client traffic, or a separate radio and/or band as used by client traffic.

As is known to the art, mesh networks may be self-organizing, and adaptive, shifting their interconnections as access nodes enter and leave the network, or other characteristics of the network or its operating environment change.

It is common to represent mesh networks as a one or more trees, with each tree rooted at a root node or mesh portal. Topologically, these trees are cycle-free. Each access node forming a tree in a mesh network therefore has one parent it reports to and zero or more access nodes reporting to it as children.

In operation of a mesh network, there may be occasions where it is useful to have all access nodes report back to the controller. This may be for status, local operating parameters, or in the operation of a sensor network, reporting sensor data collected by the nodes.

To do this, nodes in the mesh network must report back, through the mesh network. One approach to doing this is to use the already existing contention resolution mechanisms in the network and let reporting from nodes be a free-for-all.

A more orderly approach assigns a reporting schedule or sequence to nodes on the mesh network. Such a schedule based access method provides for reduced latency, reduced collisions, and lowers overhead in sending and receiving of packets. This problem may be equated to “llattening” the mesh network and assigning a reporting sequence to the nodes in the mesh.

What is needed is a way to assign a reporting sequence to nodes in a wireless mesh network.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention in which:

FIG. 1 shows a mesh network, and

FIG. 2 shows a mesh network.

DETAILED DESCRIPTION

Embodiments of the invention relate to improved methods of assigning a reporting sequence to wireless access nodes on a mesh network. In an embodiment of the invention, given one or more trees representing a mesh network, each cycle-free tree having as its root a root node or mesh portal connected to a node controller, reporting slots are assigned to nodes by having each node assess its 2-hop neighborhood to form a 2-hop neighborhood list, and ordering the nodes based on the results of a depth-first search (DFS) of the tree using the 2-hop neighborhood information, sorting the nodes based on the finish time of the DFS visit to give the post-order traversal of the nodes within a tree. For multiple trees connected to a controller, intersecting or collocated trees may choose different reporting channels, such that they do not conflict with each other, and allowing parallel transmission among nodes in different tree clusters. The slot assignment process may be repeated periodically, or on detection of events signaling a change in the topology of the mesh network, such as the loss or addition of an access node, or a change in connectivity between access nodes.

As shown in FIG. 1, network 100 such as a wired IEEE 802.3 Ethernet network is connected to controller 200. Controller 200 supports wired connections 250 to root access nodes 300 a, 300 b. These root access nodes provide wireless communications to mesh nodes 400 a, 400 b, which in turn support wireless clients such as wireless client 500. As is understood in the art, controller 200 is a purpose-built digital device having a CPU 210, memory hierarchy 220, and a plurality of network interfaces 230, 240. CPU 210 may be a MIPS-class processor from companies such as Raza Microelectronics or Cavium Networks, although CPUs from companies such as Intel, AMD, IBM, Freescale, or the like may also be used. Memory hierarchy 220 includes read-only memory for device startup and initialization, high-speed read-write memory such as DRAM for containing programs and data during operation, and bulk memory such as hard disk or compact flash for permanent file storage of programs and data. Network interfaces 230, 240 are typically IEEE 802.3 Ethernet interfaces to copper, although high-speed optical fiber interfaces may also be used. Controller 200 typically operates under the control of purpose-built embedded software, typically running under a Linux operating system, or an operating system for embedded devices such as VXWorks.

Similarly, as understood by the art, wired and wireless access nodes 300 a, 300 b and 400 a, 400 b are also purpose-built digital devices. These access nodes include CPUs 310, 410, memory hierarchy 320, 420, and wireless interfaces 330, 430. Root nodes 300 include wired interface 340. While wired interface 340 may not be present in mesh nodes 400, it may be present but not used for direct communication with controller 200. As with controller 200, the CPU commonly used for such access nodes is a MIPS-class CPU such as one from Raza Microelectronics or Cavium Networks, although processors from other vendors such as Intel, AMD, Freescale, and IBM may be used. The memory hierarchy comprises read-only storage for device startup and initialization, fast read-write storage such as DRAM for holding operating programs and data, and permanent bulk file storage such as compact flash. Wireless access nodes 300, 400 typically operate under control of purpose-built programs running on an embedded operating system such as Linux or VXWorks. Wireless interfaces 330, 430 are typically interfaces operating to the family of IEEE 802.11 standards including but not limited to 802.11a, b, g, and/or n.

Wireless client 500 is also a digital device, similarly having CPU 510, memory hierarchy 520, wireless interface 530, and I/O devices 550. As examples, wireless device 500 may be a general purpose computer such as a laptop, or may be a purpose-built device such as a Wi-Fi phone or a handheld scanner. In a general-purpose computer, CPU 510 may be a processor from companies such as Intel, AMD, Freescale, or the like. In the case of purpose-built devices, Acorn or MIPS class processors may be preferred. Memory hierarchy 520 comprises the similar set of read-only memory for device startup and initialization, fast read-write memory for device operation and holding programs and data during execution, and permanent bulk file storage using devices such as flash, compact flash, and/or hard disks. Additional I/O devices 550 may be present, such as keyboards, displays, speakers, barcode scanners, and the like.

When operating in a mesh environment, client 500 establishes a connection with a mesh node 400. Traffic from client 500 goes initially to the mesh node 400 it had established the connection with, and from that mesh node, through other mesh nodes back to root node 300, and through controller 200 to network 100.

According to an embodiment of the invention, nodes are enumerated, assigning slots to nodes in the mesh network through the following process, in which each node knows its parent, its children, and its 2-hop neighborhood, and starting with the root node:

-   -   While all children not visited do         -   1. Send token to a child node not visited         -   2. Wait until the token comes back         -   3. Remember the slot allocation of each child     -   When all nodes in the subtree rooted at the node have been         visited, assign a possible slot number s with the following         constraints:         -   1. s>maximum slot number allocated to any of its children             and         -   2. s≠slot number already allocated to a node in its 2-hop             neighborhood.

Append the node id and the slot number chosen to the token, and return the token to the parent.

As will be appreciated by those familiar with the art, this process enumerates nodes using a depth-first search (DFS) of the tree, but based on the 2-hop neighborhood information.

Referring to the mesh network of FIG. 2, the process begins with nodes 400 assessing their 2-hop neighborhoods. This is achieved, for example, by each node 400 sending a broadcast packet with a TTL of 2, and collecting all such broadcast packets the node receives as a list of 2-hop neighbors. Nodes on such lists are identified by some unique identifier, such as MAC address, although other unique identifiers may be used such as node serial numbers or the like. This list of 2-hop neighbors is different from the tree topology of the mesh network; rather, it represents a list of nodes that are in RF proximity to a node. According to the present invention, slot assignment takes advantage of the assumption that if a node is not on a target node's 2-hop list, then both nodes may transmit at the same time on the same frequency without interfering with each other.

According to the present invention, root node 300 a generates a token packet and passes it around in DFS order. The node holding the token allocates a slot for itself using its 2-hop neighborhood when the token returns to it after traversing all of its children. The node identifier and allocated slot is put in the token, and hence the token contains the allocated slots of all the nodes it has traversed. Choosing the next child in the DFS is based on the degree of the children. The higher the degree of the child, the earlier it gets the token, thereby leading to allocation of earlier slots in dense areas compared to less dense areas of the tree.

Referring again to FIG. 2, dotted lines 410 indicate nodes which share a 2-hop neighborhood. In this case, nodes 400 c, 400 d, and 400 e share the same 2-hop neighborhood, and nodes 400 b, 400 a, and 400 d share the same 2-hop neighborhood. Slots are assigned by root node 300 starting the process by sending an empty token to its first child, node 400 a. This is accomplished by sending a wireless packet from root node 300 a to mesh node 400 a. Node 400 a receives the token, and passes it to its child, node 400 c.

Node 400 c, having no children, selects a slot number according to the process previously detailed. Since the token is empty, node 400 c chooses an initial slot value, for example, 1. It appends a tuple to the packet containing its identification, which may be its MAC, serial number, or other unique identifier, together with the slot number it has chosen, and returns this packet to its parent, node 400 a, by sending the token wirelessly to node 400 a.

Node 400 a receives the token and sends it to child node 400 d. Node 400 d selects a slot number which is greater than the maximum slot number allocated to any child, and not already allocated by a node in its 2-hop neighborhood. Slot number 2 meets these criteria. Node 400 d appends a tuple containing its ID and the slot number 2 to the token and returns it to its parent, 400 a.

Parent node 400 a now applies the same selection rules and picks slot number 3, appending a tuple with its ID and slot number 3 to the token and passing it back to its parent, 300 a.

Node 300 a sends the token to node 400 b, which sends it to child node 400 e, Node 400 e chooses slot number 4, appending a tuple with its ID and slot number 4 to the token and returning it to its parent, 400 b. Node 400 b now sends the token to child 400 f. According to the present invention, since node 400 f does not have any children, and the token does not contain any nodes which are in its 2-hop neighborhood, node 400 f chooses slot number 1. While node 400 c also chose slot 1, because nodes 400 c and 400 f are not in the same 2-hop neighborhood, both nodes can transmit at the same time, using slot 1, without risk of interference. Thus slot 1 may be reused. Node 400 f appends a tuple with its ID and slot number 1 to the token and returns it to its parent, 400 b.

Under the process given, node 400 b takes as its slot number a number greater than any slot number allocated to any of its children, and not equal to a slot number allocated to a node in its 2-hop neighborhood. Slot number 5 meets both these criteria. Node 400 b appends a tuple with its ID and slot number 5 to the token and returns it to its parent, node 300 a, and the enumeration and slot assignment is complete.

Note that by allocating slots using the process detailed, relying on 2-hop neighborhoods to eliminate conflicts, slots may be reused safely.

Once nodes have been enumerated and assigned slots, these slot assignments may be used in reporting information back to controller 200.

According to an aspect of the invention, when controller 200 requests a status update through root node 300 a, for example by broadcasting a request to report status beginning at a specified time, nodes in that tree report status in enumerated slot order. As all child nodes report back to the parent node before the parent node reports back to its parent, aggregation of information from child nodes is possible. This allows parent nodes to analyze, filter, summarize, and/or compress data from children as part of the aggregation process.

In aggregating status information, a parent node for example concatenates the status information reported by its children, concatenates its own status information, and passes this aggregated status to its parent node.

Filtering or other processing may also occur during this process. As an example, in reporting minimum and maximum node temperatures over the mesh network, each node reports to its parent the minimum and maximum temperatures for itself and its children. As the slot ordering establishes that each parent node receives status from its children before it sends its status to its parent, each parent node can aggregate these minimum and maximum temperatures over its children and itself. Similarly, if controller 200 is polling for error conditions, rather than having to process a multitude of “no error” messages from mesh nodes, aggregation by parent nodes allows only error reports to be propagated back to root node 300 and controller 200.

While the invention has been described in terms of various embodiments, the invention should not be limited to only those embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is this to be regarded as illustrative rather than limiting. 

1. A method of operating a wireless mesh network having a root node connected to a controller and a plurality of mesh nodes, each node having a node id, the mesh nodes communicating in a tree topology where each mesh node has a parent node and zero or more child nodes, comprising: each mesh node determining its 2-hop neighborhood, and assigning slot numbers to the mesh nodes by performing a depth-first search of the tree using the 2-hop neighborhood.
 2. The method of claim 1 where the step of determining a 2-hop neighborhood further comprises creating a list of node ids within 2 hops of the current node.
 3. The method of claim 2 where the step of assigning slot numbers using the 2-hop neighborhood further comprises assigning a slot number to the current node which is greater than the slot numbers already allocated to any child nodes or to any nodes in its 2-hop neighborhood.
 4. The method of claim 1 where the steps of determining 2-hop neighborhoods and assigning slot numbers are repeated when the tree topology changes.
 5. The method of claim 1 further comprising issuing a request from the controller to the root node and all mesh nodes to report status in slot number sequence.
 6. The method of claim 5 where a parent node aggregates status information reported by its child nodes.
 7. The method of claim 5 where a parent node filters status information reported by its child nodes.
 8. A method of operating a wireless mesh network having a root node connected to a controller and a plurality of mesh nodes, each mesh node having a node id, the mesh nodes communicating in a tree topology with the root node, each mesh node having a parent node and zero or more child nodes, the method comprising: each mesh node calculating its 2-hop neighborhood, initiating a depth-first search of the tree starting with the root node and an empty token comprising: while all child nodes are not visited: send the token to a child node not visited wait until the token returns remember the slot allocation of each child when all nodes in the subtree rooted at the node have been visited, assign a slot number s to the node with the following constraints: s>maximum slot number allocated to any of the node's children, and s≠slot number already allocated to a node in its 2-hop neighborhood append the node id and the slot number chosen to the token and return the token to the parent node.
 9. The method of claim 8 further comprising: issuing a request from the controller causing the mesh nodes to report status in slot number order.
 10. The method of claim 9 where a parent node aggregates status information reported by its child nodes.
 11. The method of claim 9 where a parent node filters status information reported by its child nodes. 